The invention pertains generally to a solid-state image sensing device. More particularly, the invention pertains to a solid-state imaging device particularly adapted for use in an electronic still camera.
FIG. 1 illustrates a conventional image sensing device of the same general type to which the invention pertains. Pixel array 12 is formed as a matrix of individual light-sensing pixels (picture elements) arranged in rows and columns. Charges are stored in each of the pixels in a magnitude related to the intensity of the light striking the element subsequent to the last time the charge on the element was reset. More specifically, first a capacitor in each pixel is charged to a predetermined level, and then the array is exposed to image light for a predetermined duration. During the exposure time, the charge initially stored in each of the pixels is depleted in an amount which increases with the intensity of the light striking the pixel during the exposure time.
Following exposure, the output lines from a vertical shift register 11 are actuated in sequence to read out the charges stored in the pixel array 12 row by row. Particularly, a first output line 16 of the vertical shift register 11 is first activated by applying a pulse thereto. Following the activation of the line 16, a horizontal shift register 14 is operated to read out, via an output switch 13, the charges extracted from the row of the pixel array 12 actuated by the line 16. The next output line of the vertical shift register 11 is actuated, and the horizontal shift register 14 is cycled again to read out a subsequent row of the pixel array 12. This process is continued until the last output line 17 of the vertical shift register 11 has been actuated, at which time the charges of all of the pixels of the pixel array 12 have been read out, via the output switch 13, in serial order, onto a video output line 18.
FIG. 2 is a waveform diagram showing the application of the pulses from the vertical shift register 11 to the pixel array 12. As mentioned above, it is necessary to reset the pixel array 12 to a predetermined, uniform state prior to exposure. Were this not done, charges accumulated prior to exposure, such as may be due to a dark current, would interfere with the levels effected in response to the next exposure operation, hence leading to a distorted output. To reset the pixel array, it has been the practice to scan the pixel array 12 once or twice in the pattern indicated in FIG. 2 prior to exposure. However, the total time required to scan the array is long relative to typical exposure times. Hence, in a still camera, there is an appreciable delay between the time the shutter button is depressed and the time when the actual exposure is performed.
Further, because in the presence of a bright image the picture elements of the array 12 might otherwise saturate due to the fixed read-out time employed by the conventional arrangement, it was necessary to provide a mechanical shutter to control the amount of light striking the pixel array 12.
Accordingly, it is an object of the present invention to provide an image sensor and a method for operating an image sensor in which the above-mentioned drawbacks have been eliminated.